1. Field of the Invention
The present invention generally relates to apparatus and methods for completing a well. Particularly, the present invention relates to apparatus and methods for cementing operations. More particularly, the present invention relates to apparatus and methods for locating a cementing apparatus in the wellbore. More particularly still, the present invention relates to apparatus and methods for determining the amount of cement displaced.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or a liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or “hung” off of the existing casing. Afterwards, the second casing string is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
The process of cementing a liner into a wellbore typically involves the use of liner wiper plugs and drill-pipe darts. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. A liner wiper plug is typically located inside the top of a liner, and is lowered into the wellbore with the liner at the bottom of a working string. The liner wiper plug has radial wipers to contact and wipe the inside of the liner as the plug travels down the liner. The liner wiper plug has a cylindrical bore through it to allow passage of fluids.
Typically, the cementing operation requires the use of two plugs and darts. When the cement is ready to be dispensed, a first dart is released into the working string. The cement is pumped behind the dart, thereby moving the dart downhole. The dart acts as a barrier between the cement and the drilling fluid to minimize the contamination of the cement. As the dart travels downhole, it seats against a first liner wiper plug and closes off the internal bore through the first plug. Hydraulic pressure from the cement above the dart forces the dart and the plug to dislodge from the liner and to be pumped down the liner together. At the bottom, the first plug seats against a float valve, thereby closing off fluid flow through the float valve. The pressure builds above the first plug until it is sufficient to cause a membrane in the first plug to rupture. Thereafter, cement flows through the first plug and the float valve and up into the annular space between the wellbore and the liner.
After a sufficient volume of cement has been placed into the wellbore, a second dart is deployed. Drilling mud is pumped in behind the second dart to move the second dart down the working string. The second dart travels downhole and seats against a second liner wiper plug. Hydraulic pressure above the second dart forces the second dart and the second plug to dislodge from the liner and they are pumped down the liner together. This forces the cement ahead of the second plug to displace out of the liner and into the annulus. This displacement of the cement into the annulus continues until the second plug seats against the float valve. Thereafter, the cement is allowed to cure before the float valve is removed.
During the cementing operation, it is desirable to know the location of the second plug/dart in the wellbore. Generally, the position of the plug will indicate the amount of cement that has been displaced into the annulus. If insufficient cement is displaced (“underdisplacement”), cement will remain in the casing. If too much cement is displaced, (“overdisplacement”), portions of annulus will not be cemented.
One method of determining the plug location is by measuring the volume displaced after the second plug is released. Then, the volume displaced is compared to the calculated displacement volume based upon the dimensions of the casing or drill pipe. A second method is attaching an indication wire to indicate that a plug has been released. The indication wire is usually 2 to 3 feet in length. A third method is using mechanical flipper indicator. In this method, a lever is disposed below the plug container. A released plug will shift the lever when the plug travels by it. A fourth method is using electromagnetic or magnetic signals. Generally, an identification tag is attached to the plug or dart. A detector located below the cementing head picks up the signal when the plug passes to indicate that the plug has been launched.
There are drawbacks to using these methods to determine plug location. For instance, the displacement method is not very accurate and does not give a positive indication that the plug is moving at the same rate as the fluid being pumped behind the plug. Casing and drill pipe are generally manufactured to dimensional tolerances that could result in a substantial difference between the calculated displacement volume and the actual displacement volume. Further, fluids are subject to aeration and compression during the operation, thereby affecting measured volume. Indicator wires and mechanical flipper indicators only indicate that the plug has been released, not the location thereof. Finally, the signal detectors cannot track the plug for long distances and only indicate that the plug has moved past the detection device.
There is a need, therefore, for an apparatus for locating a plug in the wellbore. Further, there is a need for an apparatus for determining the amount of cement that has been displaced. The need also exists for a method for completing a cementing operation.